Wireless communication systems are known to include a communication resource controller, a plurality of communication units, and communication resources. In such communication systems, the communication units communicate with each other via a communication resource, where a communication resource may be an RF channel, a pair of RF channels, a TDM slot, or any medium for carrying RF signals. To initiate a communication, a communication unit transmits a communication request to the communication resource controller via a communication resource dedicated for control information (control channel). The communication request includes the identity of the requesting communication unit, the identity of a target communication unit, or units, and a request for a particular service, such as allocation of a communication resource. For example, the request may be for a group call which identifies all the communication units in the same group as the requesting communication unit.
Upon receiving this request, the communication resource controller determines whether the initiating communication unit has a valid unique identification code (unique ID) and is therefore allowed to access the system. If the communication unit's unique ID is valid, the communication resource controller grants the request. Having granted the request, the communication resource controller allocates a communication resource to the requesting communication unit and the target communication units, and then transmits a communication resource allocation message on the control channel.
While the communication resource controller performs a validity check of the communication unit's identity, i.e., the unit's unique ID, the controller does not verify that the communication unit received its unique identification code in an authorized manner. Typically, the unique ID is permanently programmed into the communication unit by the manufacturer prior to delivery of the communication unit. Upon delivery of the communication system, a system manager enters the unique ID of all the communication units purchased for the system into the controller, thus rendering these unique IDs valid. Because the manufacturer permanently programs the unique ID into the communication unit and the system manager programs the controller, it assumed that a valid unique ID used by a communication unit was authorized by the system manager. Unfortunately, this is not always the case. To illustrate, assume that a communication system "hacker" has knowledge of valid unique IDs for a particular system. The hacker can program these unique IDs into other communication units, such that these communication units have valid unique IDs and thus have access to the communication system, however, the unique IDs were obtained in an unauthorized manner. When these "unauthorized" communication units request access to the system, the controller verfies the unique ID and allows access.
As is readily apparent, when unauthorized communication units access a communication system, the authorized communication units suffer. They suffer because communication resources are less readily available and because the unauthorized communication units can interfere with important "authorized" communications. For example, assume that the authorized communication units are operated by a city's police force, thus most communications are important to public safety. Further assume that an "unauthorized" communication unit has received a valid unique ID from a hacker, or has stolen an authorized communication unit, and is interfering with the police communications. The person operating the unauthorized communication unit could send police officers on "wild goose" chases, or say he/she will respond to an emergency call, but doesn't. If this were to happen, disastrous results could occur.
To combat this, several techniques have been developed to detected the use of unauthorized communication units. One such technique determines the geographic separation between successive transmissions using the same unique ID within a given time period. If the geographic separation is excessive for the given time, it is assumed that more than one communication unit is using the same unique ID. Another technique counts the number of times a unique ID is used during a given time period. If the number of request is excessive, it is assumed that more than one communication unit is using the same unique ID. When multiple use is detected, a system manager is notified. The system manager may disable the communication units, thus preventing unauthorized use.
While these techniques work well to detect unauthorized use, they provide little information as to the location of the person operating the unauthorized communication unit. Ascertaining the location of such a person would be most helpful in retrieving a stolen communication unit, in apprehending the person, and preventing further interference with authorized communications. Therefore, a need exists for a method that detects the location of an unauthorized communication unit.